1. Field of the Invention
The present invention relates to a method for reducing audio noise of a display. More particularly, the present invention relates to a method for reducing audio noise of a liquid crystal display (LCD).
2. Description of Related Art
Generally, an LCD device drives each scan line on a display region through inversion driving manner, thereby inverting the scan line and thus eliminating a problem of DC bias residue on the LCD screen. Practically, voltages with opposite polarities are applied on the same scan line of neighboring frames to drive the liquid crystals. In order to achieve the objective, the driving circuit must invert the driving voltage and the substrate voltage of the LCD screen at the same time.
FIG. 1 is a schematic timing diagram for driving a conventional LCD. As shown in the figure, in order to invert the polarity of the driving voltage of the scan line, the substrate voltage VCOM must be inverted following the horizontal synchronization signal (HSD), and the transition timing is at a timing when an output enable signal OEV is at a high level status. The horizontal synchronization signal HSD has a frequency of 15 kHz, so it can be clearly known from FIG. 1 that the frequency of the substrate voltage VCOM is 7.5 kHz.
However, during the inversion driving procedure, that is, when the substrate voltage is inverted, the frequency of the substrate voltage VCOM becomes 7.5 kHz. The substrate voltage signal VCOM with the frequency of 7.5 kHz can generate resonance with the material on LCD screen module, such that an audio noise with a frequency of 7.5 kHz is generated. As shown in FIG. 2, it can be seen that under the frequency of 7.5 kHz, the audio noise reaches up to 22 dB.
The intensity of the audio noise has already reached up to such a degree that human beings can hear. As for those ordinary small-size mobile devices, for example, mobile phones, the LCD screen is close to the user's ear during the call, so the audio noise may cause troubles when the user is listening to the sounds from the phone, and as a result, the communication quality is deteriorated.
In order to solve the problem, in the conventional art, various methods of changing the scanning time for the scan line and reducing the charging time are used, but as a result, the display quality is affected or the cost for altering the circuit design is increased. Therefore, how to reduce the influences caused by the audio noise to the user in a more efficient and cost-effective way has become an urgent issue to be solved in this field.